Compressed gases such as compressed air, nitrogen, oxygen, helium, etc. are used extensively throughout a wide spectrum of industries such as automotive, chemical, electronic, aerospace, oil refining and health care industries. There is often a need in these industries for compressed gas which is free of contaminants such as particulate matter, aerosols (liquid droplets), water vapor, oil vapor or other undesired agents. For example, if used to spray paint, it is important that oil, water and particulate matter be removed from the compressed air to prevent the paint from becoming contaminated, thereby ruining the paint finish. Similarly, if compressed air provides a source of breathing air, for example, by SCUBA divers, firemen, air crew members or hospital patients, it is critical that particulate and oil contaminants be removed from the air to prevent ingestion of the contaminants into the lungs of the person.
Gas compressors themselves are often the source of contaminants in compressed gas. Air drawn into a compressor, for example, is often not properly filtered at the compressor intake, thus allowing particulates commonly in the air, such as pollen, smoke, and dust, to be drawn in and introduced into the compressed air supply. The moving parts of compressors such as pistons or impellers are lubricated with oils which can vaporize, atomize, oxidize or otherwise breakdown due to agitation by the moving parts and the increase in temperature associated with compressing a gas. The lubricating oil or its breakdown by-products become entrained in the compressed gas either as an aerosol (oil droplets suspended in air) or a vapor (oil in the gaseous state) as the gas leaves the compressor. Further, water is a natural component of air, and unless deliberately removed, water will comprise a natural part of compressed air as well in both aerosol (water droplet) and vapor (gaseous) form.
Commonly, filters are used to remove such undesirable particulates from the gas. For example, Haramoto et al. U.S. Pat. No. 5,910,165 discloses a dryer in which desiccant is used as a filtering material to remove water from compressed air which is passed through the desiccant. In such filters, desiccant which changes color when saturated (e.g., from blue to pink) has also been used to indicate when the filter should be replaced. When the filter desiccant has changed almost fully from, for example, blue to pink, that is an indication that the desiccant (i.e., filter cartridge) should be replaced.
Often, filters have been enclosed in a pressure vessel and thus the filter is not visible unless the pressure vessel is opened. In that case, it is necessary for the user to periodically check the status (color) of the desiccant to see if it needs replacement, often resulting in unnecessarily early replacement (and wasting) of desiccant when the user sees that the desiccant is approaching saturation, because the user may worry that he will neglect to check the desiccant again until after it reaches saturation. In some filters (e.g., Brown et al. U.S. Publ. No. 2008/0005919 and Flynn et al. U.S. Pat. No. 5,846,271), a translucent housing has been used to enable the user to see color changes of the desiccant to know when to change the desiccant without requiring disassembly. In still other structures, indicators have been installed in the air line itself downstream from the filters which detect and in some manner indicate that undesirable particles (e.g., water) are still in the compressed gas even after passing through all the filters, such as by use of color changing desiccants.
Moreover, multiple stage in-line filters in which the above described filters may be one stage have also been used (with, e.g., stages between the gas compressor and the compressed air output, such as a paint spray painter) with different stage components used to filter different particulates. Rhodes U.S. Pat. No. 3,791,105, Daniels U.S. Pat. No. 4,822,387 and Flynn et al. U.S. Pat. No. 5,403,387, for example, show such multi-stage filters, each stage having a separate housing/pressure vessel.
Single cartridges have also been provided with multiple filtering materials, such as activated charcoal for filtering oil particles and desiccant on opposite sides of a permeable partition such that, for example, the compressed gas first passes through the charcoal to remove oil particles and then through the partition to the desiccant where water is removed. Brown et al. U.S. Publ. No. 2008/0005919 and Flynn et al. U.S. Pat. No. 5,846,271 disclose such cartridges.
In all instances, it is desirable to be able to determine when the filter material needs to be changed without requiring constant disassembly to check the status of the material, including instances in which pressure vessels are used which are not translucent.